The present invention relates to linear position encoders generally. The invention has particular although not exclusive relevance to non-contact linear position encoders. The invention is particularly suited for use in systems where there may be electromagnetic interference, particularly when used in automotive applications.
Many types of non-contact linear position sensors have been proposed. A system similar to the present invention is described in U.S. Pat. No. 5,815,091 which is incorporated herein in its entirety by reference. In particular, U.S. Pat. No. 5,815,091 discloses a system for use as a linear position encoder. The basic layout of U.S. Pat. No. 5,815,091 is illustrated in FIG. 1. FIG. 1 shows an outside trace 16 which is connected to a signal generator 11 that generates a trace signal typically in the 0.1 to 10 MHz. range. This outside trace 16 becomes the excitation trace. When an excitation signal is generated in the excitation trace, an output from a pair of phase quadrature conductive windings 13 and 15 depicted as sine and cosine traces is zero volts if perfect symmetry is observed. When a circuit 10 which is resonate at the excitation frequency is placed over the circuit board having the sine and cosine traces the symmetry is distributed and signals are induced into the sine and cosine traces. The voltage level of the signals at the outputs of corresponding sine and cosine traces are the sine and cosine representative of the linear position of the resonate circuit 10 with respect to the stationary printed sine and cosine traces.
The system determines the position of the movable element (i.e., resonant circuit) relative to the stationary element (i.e., circuit board) by utilizing the variation in mutual inductance between the coil and the plurality of sine and cosine wave windings. More specifically, when the power source energizes the coil, a large voltage signal is induced in a sine and cosine wave windings if the coil is adjacent a high part thereof. Only a small voltage signal is induced in a winding if the coil is adjacent a low part thereof. Therefore, the
However, this system has a number of disadvantages that pose real world problems when implemented for use, particularly in automotive applications. Firstly, the system is not “balanced”, i.e. it is not immune to electromagnetic interference. The flux from the excitation loop trace easily interacts with conductive materials in its proximity.
Secondly, the ability to get a null or zero signal at the outputs of the sine and cosine traces without the resonate circuit present varies with the mounting conditions and nearby objects. Thirdly, the resonate frequency of the moveable board will change with temperature and the presence of nearby conductive objects with respect to the excitation frequency which will greatly change the induced signal.